T - Factary

Temperature

Although these days it’s a relatively simple
thing to measure the temperature of anything,
it is quite difficult to define exactly what temperature
itself is.

We know from experience that hot things (from
the oven say) will eventually cool down and that
cold things (from the freezer) will warm up. What
is happening is that in both cases the object
is reaching equality (technically it is called
thermal equilibrium) with its surroundings (the
air around it). We can therefore define temperature
as that quantity which is the same for both things
(the initially hot/cold object and the air) when
they have settled to thermal equilibrium.

There have been many different scales and devices
created for measuring temperature. The three most
commonly used today are the Fahrenheit,
Celsius
(or Centigrade) and Kelvin
scales.

One
of the first attempts to make a standard temperature
scale happened about AD 170. Galen, in his medical
writings, proposed a standard "neutral"
temperature made up of equal quantities of boiling
water and ice. On either side of this temperature
were four degrees of heat and four degrees of
cold. Crude maybe, but at least it was an attempt
to make heat measurable.

The earliest devices used to measure temperature
were called thermoscopes. They consisted of a
glass bulb having a long tube extending downward
into a container of coloured water.

So where did all this Celsius, Centigrade, Fahrenheit
and Kelvin stuff come from?

It's a tale that starts with three scientists;
a Dane, a Dutchman and a Swede at around the beginning
of the 18th century and ends with a Belfast-born
'Scot' in the middle of the 19th century. Very
cosmopolitan. Their names were Roemer, Fahrenheit,
Celsius and Thomson. It begins to sound familiar.
So were Roemer and Thomson the only ones to miss
out in historical terms because I've never heard
of Roemer or Thomson degrees? Not really. Roemer
had other strings to his bow, as they say, and
Thomson changed his name.

Roemer was an astronomer and was the first person
to measure the speed of light by observing the
satellites of Jupiter - not an insignificant contribution
to science.

However he was also one of the first to successfully
produce a "temperature measuring thingy"
(a thermometer to you and me). He created a glass
tube with a glass bulb at the end - pretty much
as we have today. He filled it with strong wine,
coloured with saffron to make it easier to see
or less likely to be drunk? He knew that temperature
could be measured by recording the volume of the
liquid by seeing how far up the thin tube the
liquid rose when heated to different temperatures
(liquids expand when they are heated). In practice
he took a measurement of freezing water, another
of boiling water and then divided the scale into
seven parts. He added an eighth part, as big as
the others, below the freezing point and called
it zero degrees. His boiling point was at 60 degrees.

Why would he choose 60 degrees as the top end?
Could it have been anything to do with there being
60 minutes in an hour?

Sixty is actually a pretty convenient number
for doing simple arithmetic. It is easy to calculate
fractions of it because it has 10 factors (2,3,4,5,6,10,12,15,20,30)
- numbers which can divide into it exactly. However,
50 only has 4 factors and 100 only 7. That probably
helps explain why there are 60 minutes in an hour
and so Roemer was just following common sense
tradition.

So now he had a scale divided into 8 parts going
from 0 to 60 - so each part was 7.5 degrees. The
freezing point of water was at 7.5 degrees. On
this scale his experiments showed that human body
temperature was pretty constant and equal to about
22.5 degrees.

Enter Mr Fahrenheit. Roemer showed him one of
his new thermometers which used his temperature
scale but only measured up to about the temperature
of the human body, which on his scale remember
was 22.5 degrees. Building such a device makes
sense - after all how often do you need to measure
temperatures higher than that? Fahrenheit realised
the potential of Roemer's device but didn't like
fractions - who does? So he set about refining
it. First of all he divided Roemer's scale more
finely (to get rid of the fractions) so that one
degree in Roemer's scale now equaled 4 new Fahrenheit
degrees. He reset the zero degrees to be exactly
where a mixture of ice, water and salt freezes.
I suppose that was about the coldest thing he
could easily produce and so now he had a scale
from 0 (ice, water, salt) to 90 (human body, =
22.5 x 4) degrees, with pure water freezing at
30 (= 4 x 7.5) degrees.

He was happy with that for a while but eventually
figured that mercury was a better liquid to deal
with in these devices rather than Roemer's alcohol,
which was of variable quality from thermometer
to thermometer, and also he didn't like dividing
by 3 rather than 2 so he reset the scale again
so that water froze at 32 degrees and the human
body temperature was 96 degrees.

After Fahrenheit's death it was realised that
human body temperature can vary and is therefore
not a good scale point so once again the boiling
point of water was used and set to 212 degrees
(close to Fahrenheit's original value of 205 degrees)
for convenience of dividing the scale.

Meanwhile, a fresh start had been made by the
Swedish scientist Celsius - he had no hang-ups
about the number 60 and built a device where there
were 100 degrees between the freezing and boiling
points of water. Sounds an obvious thing to do,
right? All the best ideas sound obvious afterwards.

One final twist in the tail - Celsius actually
set freezing point at 100 degrees and boiling
at 0 degrees. All scientists can be mad once in
their lives. That was reversed after his death
to be the more natural way around we know today
and in 1948 the temperature scale was officially
named the Celsius scale (the term centigrade had
come into common use) at the Ninth General Conference
of Weights and Measures.

So where does Thomson come into all of this?
William Thomson was born in Belfast in 1824, but
his family moved to Glasgow when he was six. He
went to Glasgow university at the ripe old age
of 10, still a record. After doing some fundamental
work on heat and temperature he devised a temperature
scale similar to that of Celsius. However, his
scale started at -273.16 °C. There was a very
good reason for that. He had worked out that that
was the temperature at which matter (molecules
and atoms)
would have no kinetic energy and therefore no
movement. For this reason this temperature is
referred to as 'absolute zero' and is the coldest
anything in the universe can get.

Notice that the unit of temperature on Kelvin’s
scale is one kelvin NOT one degree, so it is different
in that respect from Celsius and Fahrenheit. The
correct expression is therefore, ‘the temperature
of the Sun’s surface is about 6000 kelvin’
not ‘the temperature of the Sun’s
surface is about 6000 degrees kelvin’.

For his work in physics, Thomson was knighted
and eventually created Baron Kelvin of Largs,
which is where the unit of temperature gets its
name.

So there you are, you can take your choice and
say:

'Phew it's hot today, it's 89 degrees Fahrenheit'

'Phew it's hot today, it's 32 degrees Celsius'

'Phew it's hot today, it's 305 kelvin'

Tera (t)

A prefix indicating one million million, or one
thousand billion, of something. In numbers that
is 1,000,000,000,000 or 1012

When used before a unit abbreviation, it is abbreviated
to ‘t’. For example tW indicates a
terawatt - that's a very big light bulb.

One common use of the prefix tera today is in
Information Technology where very large computer
memory or disk space is discussed e.g. a terabyte
of memory. But (as ever) we have to be a little
careful here. Kilo, mega, giga and tera are all
prefixes that normally refer to powers of 10;
a kilometre is 1000 or 103 metres. However computers
and computer people work in powers of 2. So in
computer-speak a kilobyte is 210 (=1024) bytes
rather than 103 (=1000). The bigger the numbers,
the bigger the difference. By the time we reach
'terabyte' we might think we are talking about:

1012 = 1,000,000,000,000 bytes

but actually the number of bytes will be

240 = 1,099,511,627,776 bytes (nearly
10% more)

Can you work out the difference between the expected
and actual size of a petabyte?

Thermonuclear Fusion

The process by which the nuclei of atoms can
join together so that the mass of the joined-up
nucleus is slightly less than the mass of the
parts that went into it. The energy originally
locked up in the mass that is ‘lost’
is transferred into radiation and particle energy
by the famous E=mc2 equation.

In the Sun, atomic nuclei (nucleuses) of hydrogen
combine to form helium. The same process happens
in a hydrogen bomb. Many laboratories are also
trying to make fusion work in a controlled way
(bombs are generally considered to be uncontrolled).
If this succeeds it will provide a very convenient
and pollution-free source of energy.

The word fusion is used in biology as well. It
is used to describe the event when two cells combine
(or stick) together.

When things 'split apart' the word 'fission'
is used. When atomic nuclei split (the famous
'splitting the atom') energy is also given out
in vast amounts. This is called 'Thermonuclear
Fission'. Uncontrolled, it is the way in which
an atomic bomb works. An atom bomb is used to
set off a hydrogen bomb (which uses fusion) because
you need very high temperatures to get the nuclei
fusing together.

More
peacefully, controlled nuclear fission is the
way in which our present atomic power stations
work. The energy given off is used to boil water
into steam, which in turn drives the electrical
generators.

Thomson, Joseph John (1856-1940)

English mathematician and physicist who graduated
from Cambridge before working at the Cavendish
laboratories. Most of his work was in the field
of electricity, but he is most famous for discovering
the electron. He also deduced a surprisingly accurate
estimate of its mass (1/2000th the mass of a proton
- now known to be 1/1836th). In 1906, he received
the Nobel prize for Physics for this discovery.

Thomson,
William (1824-1907)

William Thomson (aka Lord Kelvin)

Famous physicist and mathematician born in Belfast
who was brought up as a strict Presbyterian. He
went to Glasgow university from the age of 10
and studied astronomy, chemistry and natural philosophy
(physics). William entered Cambridge university
at the age of 17. He graduated from Cambridge
with the second highest honours in his year: quite
an achievement. Eventually he became professor
of natural philosophy at Glasgow university where
he had a notorious and marathon letter-writing
exchange with Stokes, another famous scientist
of the time, over the similarities between heat
and fluids. This lead William to come up with
the scale of temperature for which he is most
famous, the Kelvin
scale. It was called the Kelvin scale because
William was later appointed Lord Kelvin by Queen
Victoria.

TRACE

This is the name of a satellite launched into
Earth orbit in April 1998. It studies the Sun's
outer atmosphere by taking very detailed images
at several wavelengths. The acronym 'TRACE' stands
for Transition Region
And Coronal
Explorer.

Transition Region

A region in the Sun's atmosphere between the
chromosphere
and the corona.
Since the Sun's temperature
rises from 10 000 oC to over 1 000
000 oC through this region, it's called
the Transition Region because it's where the temperature
is in transition from fairly cool to very hot.

Transverse Wave

In a transverse wave the particle displacement
is perpendicular to the direction of wave propagation.

In this demonstration (by Dr. Dan Russell Kettering
University Applied Physics) you can see the wave
travelling through the medium, but if you concentrate
on an individual dot in the medium, you’ll
see that it doesn’t go anywhere.